[SOLUTION] This is how I fixed the 401 error. Undiscord works again and I can delete messages now.

[gaveitatry2]

I wanted to post this in the Issues section because a lot of people only click the Issues tab and never click on the Discussions tab. But for some reason I am unable to create a new Issue. Did victornpb ban me from creating new issues or something?

This is the solution...

By the way, I use Mozilla Firefox.

I went to about:logins which pulls up the Passwords manager.
I searched for and deleted my passwords for Discord.
Note: It is important to do this because for some stupid reason Undiscord will use your Discord password as the Authorization Token instead of your actual Authorization Token.
Note: If you are not using Mozilla Firefox, then just go to the Passwords manager for your browser and delete your Discord password from there.

I refreshed the Discord page (to make it easier later).
I pressed F12 to pull up Developers Tools.
I clicked on the Network Tab.
I clicked on the XHR tab underneath.
I typed "api" (without the quotation marks) in the filter box to narrow the search results.
I clicked on one of the search results.
I clicked on the Headers tab.
I scrolled down until I saw Authorization: (some long string of letters and numbers).
I right-clicked that and selected Copy Value.
I then opened up a Notepad document and pasted the value in there to make sure that I did in fact copy the Authorization token.
Note: I had to manually get my Authorization Token using this method, because pressing the Fill button just gave me an error message and did not work.

I then opened up the Undiscord window and pasted the Authorization Token that I got from the above steps.

And it worked. I was able to delete messages.